Light-emitting diodes (LEDs) are good candidates to replace incandescent and other light sources. LEDs have higher power to light conversion efficiencies than incandescent lamps and longer lifetimes. In addition, LEDs operate at relatively low voltages, and hence, are better adapted for use in many battery-powered devices. Furthermore, LEDs are point sources, and hence, are better adapted than fluorescent sources for lighting systems in which a point light source that is collimated or focused by an optical system is required.
To compete with incandescent lights, the output spectrum of the LED must be altered to provide a spectrum that is perceived as being “white” by a human observer. In general, LEDs generate light in a small band of wavelengths. Hence, to build a light source that is perceived as being white, light from a monochromatic LED is typically down converted by a phosphor layer to provide light in additional regions of the visual spectrum. The most common form of white LED utilizes a blue-emitting LED and a layer of phosphor that converts part of the blue light into yellow light. The combination of blue and yellow light is perceived by a human observer to be white if the ratio of blue to yellow light is properly chosen.
The color temperature of the white light source depends critically on the ratio of blue light to yellow light in the output of the light source. The amount of yellow light that is produced depends on the peak wavelength of the underlying blue light source. If the wavelength shifts, the fraction of the blue light that is converted to yellow light by the phosphor also shifts, and hence, the perceived color temperature of the light source shifts.
One of the major limitations to mass adoption of LED light sources is the ability to provide constant color and constant flux. At present, there is considerable variation both in the peak wavelength and radiant power of the LEDs. The peak wavelength and total radiant power of LEDs can vary by ±10 nm in peak wavelength and by ±20percent in radiant power. As a result, there is considerable variation in both the color temperature and radiant power of the final white LEDs unless compensating measures are taken. For example, in one solution to this problem, the LED chips are measured and sorted into bins according to the peak wavelength and the output power. A manufacturer then adjusts the manufacturing recipe to match a particular bin.
In another compensating strategy, white sources are combined and regulated in intensity to provide a white source of a predetermined color temperature and intensity. For example, U.S. Pat. No. 7,568,815 describes a scheme in which three white LEDs having different spectra are used to construct a white light source that has a particular color temperature by adjusting the relative intensities of the three white LEDs. While this approach provides a light source in which the manufacturing recipe does not need to vary due to variations in the LEDs, the cost of providing a controller and adjusting the relative outputs of the LEDs significantly increases the cost of the light source. Accordingly, a light source design that does not require a different recipe for each LED bin, while avoiding the cost of providing a controller and adjusting the output of each LED is needed.